1. Field of the Invention
The present invention relates to plugs for use in subterranean wells. More particularly, the invention relates to plugs used to block the flow of production fluid through the flow bore of a tubing string in a well.
2. Background of the Related Art
In well completion, it is often necessary to block the flow of liquids in the flow bore of a tubing string in order to isolate the upper portion of the tubing string from production fluids. Some tubing plugs are retrievable and are typically run into the tubing on coil tubing or cable and are then removed in the same way. Other tubing plugs are installed between adjacent pieces of tubing and lowered into the wellbore with the tubing string. Removal of these plugs requires either that the entire tubing string be pulled from the well or that the plugs be remotely opened when fluid flow through the flow bore is desired.
One type of plug installed between pieces of tubing string includes a central frangible element that can be either pierced or smashed by mechanical means. An example includes a one-piece frangible ceramic sealing element which, after use, is shattered by impacting with a tooth-faced, blind box hammer under force of gravity. In each of these cases, the remaining pieces of the seal must be washed out of the wellbore with completion fluid or the like making these designs unsuitable for many customers. Additionally, some designs which use a mechanical impact means to destroy the flow blocking element require an additional tool run on wire line or coil tubing to lower and then remove the impact means.
Other plugs installed between pieces of tubing are opened remotely through precise pulses of pressure which either destroy the seal element or actuate some valve located on the plug, thereby opening the sealing surface to flow. In still other instances, the plugs are destroyed with an explosive detonation also leaving bits of debris in the well which must be removed.
Also known in the art are temporary plugs made with a compressed mixture of salt and sand. These plugs may be rapidly dispersed, essentially in their entirety, by exposure of the salt and sand mixture to a wellbore fluid. However, these systems generally have been configured to block pressurized fluid from only one direction, usually downward, from the earth""s surface and are therefore useful only in one direction. Another known plug assembly includes the plug member which has a frangible or dome-shaped portion shaped in a arcurate fashion, whereby one side of the plug presents a convex surface and another side presents a concave surface. The dome configuration of these plugs typically causes the plug member to be significantly more resistant to pressure from its convex side than its concave side. Consequently, these plugs are also practically capable of blocking fluid pressure from only a single direction.
From the foregoing it can be seen that it would be desirable to provide a plug which can be installed between pieces of tubing and which can be remotely opened without leaving debris in the wellbore. Additionally, it would be desirable to provide a plug which can be opened remotely without the use of special tools either at the earth""s surface or lowered into the wellbore to the plug. Additionally, it would be desirable to have a plug which can be opened without the use of explosives or complicated pulses of pressure from the earth""s surface. Finally, it would be desirable to have a dome-shaped plug which effectively withstands pressure from two directions and does not present a threat of destruction if significant fluid pressure is placed on its concave side.
A bi-directional plug is provided which blocks the flow of fluid through a flowbore of a tubing string and can be opened by simple mechanical means, requiring no external tools. In one aspect of the invention, a piston is slidably mounted in a piston housing and fixed in a first position with a plurality of shear screws. A chamber formed within the piston housing creates a pressure differential causing the shear screws to break when a, certain pressure is applied to the surface area of the piston. When the shear screws fail, the piston accelerates towards an extended position and strikes a scored, dome-shaped plug. The piston penetrates the plug, permanently pressing and housing pieces of the plug against the wall of a plug housing and opening the flow bore of the plug to fluid in either direction.